BALTIMORE – In case it wasn't evident before opening day, we now know why the Orioles added another slugger to a team that led the major leagues in home runs last season.

Nelson Cruz celebrated his Orioles debut by hitting a tie-breaking home run in the seventh inning, and Baltimore beat Jon Lester and the defending World Series champion Red Sox 2-1 Monday.

Signed as a free agent in February, Cruz lined a solo shot into the left field seats to put the Orioles ahead. He hit 27 homers last year with Texas during a season marred by a 50-game suspension from Major League Baseball in the Biogenesis drug scandal.

Cruz joins a batting order that features Chris Davis, who led the majors in 2013 with 53 homers and 138 RBIs. Davis went 0 for 3 with two strikeouts as the cleanup hitter, but it didn't matter because the guy in the 5-hole picked up the slack.

"It takes some of the pressure off everybody in the lineup knowing that you can pass the baton and the next guy can pick it up," Davis said. "We have to know as hitters going into the game that we can be patient, we can wait for our pitch, knowing that the guy behind us can pick us up."

Cruz, meanwhile, couldn't have been more delighted. Not only did the Orioles provide him with a fresh start, but the fans chanted "Cruuuuuz!" every time his name was announced.

"It was really neat, it was special," he said. "I made the right call to come and be part of this organization, be part of this town."

Cruz wasn't the only player to enjoy a successful start with his new team. After missing two seasons recovering from knee surgery, Boston center fielder Grady Sizemore marked his return from a 922-day absence with a second-inning single for his first hit since September 2011. The three-time All-Star for Cleveland later hit his first home run since July 15, 2011, which also came at Camden Yards.

"Today was very exciting," Sizemore said. "I couldn't wait to get to the ballpark."

Zach Britton (1-0) got the win with two scoreless innings of relief, and Tommy Hunter got three outs for the save. Hunter earned the job as closer this spring after the Orioles dealt away Jim Johnson during the offseason.

The Red Sox got two on with one out in the ninth before World Series MVP David Ortiz hit a flyball, and Jackie Bradley Jr. took a called third strike.

"It's life in baseball," Hunter said. "You've got to earn everything you get. One-run game to start the season off against the defending world champs."

Starting on the road for the fourth year in a row, the Red Sox got nine hits but stranded 12 and went 0 for 10 with runners in scoring position.

"We had a lot of opportunities, we had a lot of guys on," second baseman Dustin Pedroia said. "We just hit the ball right at people. It was one of those deals where we couldn't find a way to push runs across."

Making his fourth straight opening day start for Boston, Lester (0-1) gave up two runs, six hits and a walk in seven innings. He struck out eight.

Orioles starter Chris Tillman allowed one run, seven hits and a walk in five innings. The right-hander went 3-1 against Boston last year, but needed 104 pitches to get 15 outs Monday.

Game-time temperature was announced at 60 degrees, but a stiff wind chilled many in the crowd of 46,685 – the 23rd consecutive opening day sellout at Camden Yards.

Hopeful that the offseason acquisitions of Cruz and right-hander Ubaldo Jimenez will enable Baltimore to return to the playoffs after missing out last year, Orioles fans were out in force. Until recently, games against Boston often drew as many Red Sox backers as those rooting for the home team.

"Our crowd was legit," Hunter said. "That was awesome."

The Orioles used a walk, a single by Matt Wieters and a double-play grounder to take a 1-0 lead in the second.

The Red Sox put runners at second and third with two outs in the third before Tillman struck out Mike Carp. In the fourth, Sizemore hit a 3-1 pitch over the right-field wall.